Chemistry - How are the boiling points of Tungsten and other metals determined and proved?

Solution 1:

According to the 1913 paper THE VAPOR PRESSURE OF METALLIC TUNGSTEN, The Physical Review, vol. II, pp. 329-342:

The vapor pressure (p) of a pure substance is related to its evaporation rate (m) in a vacuum by the following relationship:

$$ m= \sqrt{\frac{M}{2\pi RT}}p$$

Evaporation rates were measured at temperatures in the range from 2400K to 3100K, heating by electric resistance of a filament, and determining the evaporation rate by weighing the filament before and after the evaporation.

The temperature was determined by observing the color and intensity of the light emitted by the filament. (Paper says they will publish a separate paper about the details of the temperature determination technique).

Vapor pressure was extrapolated to a boiling point of 5110 K using the Clausius-Clapeyron relationship.

Solution 2:

You certainly cannot measure >6000 K (Kelvin I am using SI units here) using a thermocouple because your probe would instantly evaporate itself. However, you can use the electromagnetic radiation spectrum to measure the temperature of the element. The radiation spectrum of a black body radiator is very well understood and can be measured using a spectrophotometer. Once you have the radiation specrum, you can use Planck's law of black-body radiation to calculate the temperature.

What you then need to take care of is that you melt/evaporate the element of interest and not the container and that you do not burn the element instead of vaporizing it. In case of tungsten, this can be achieved in a setup similar to a xenon lamp. A xenon lamp is nothing else than a glass container filled with inert xenon gas in which a tungsten wire is suspended and electrically connected. By carefully measuring the electromagnetic spectrum while increasing the voltage until the tungsten wire melts you should be able to measure the melting temperature. I am not sure if this also works to measure the evaporation temperature because the wire might disconnect from your power source once molten (commonly referred to the lamp being burnt out) It may be possible to shape the wire such that it does not disconnect from your power source while melting and that the heat dissipates fast enough at the electrical connections to avoid damaging those. Alternatively (with todays technical means) one could use a laser to heat the tungsten until it evaporates.